1. (Technical Field of the Invention)
This invention relates to a method of preventing the meandering movement of a printing sheet in an electrophotographic apparatus.
2. (Prior Art)
The operation of an electro-photographic apparatus such as a laser beam printer will be described with reference to FIG. 2. A printing sheet 201 is conveyed by means of upper and lower tractors 204 and 203 at a speed equal to the peripheral speed of a photo-sensitive drum 202. A toner image (not shown) is transferred onto the printing sheet 201 by means of a transfer unit 210. Thereafter, tension is given to the printing sheet 201 by a buffer as required. Under this condition, the printing sheet 201 together with the toner image is heated and pressurized by means of a heating roll 207 and a pressure roll 208 so that the toner image is fixed to the printing sheet 201. The printing sheet 201 thus treated is pulled under a predetermined tensile force by a puller 213 and a pressure roll 212, so that it is delivered into a stacker (not shown).
As is apparent from the above description, in the above-described operation, the pin drive provided by the upper and lower tractors 204 and 203 and the frictional drive provided by the heating roll 207 and the pressure roll 208 are utilized for conveyance of the printing sheet 201. When the printing sheet 201 is conveyed by the frictional force provided by the heating roll 207 and the pressure roll 208, since no means for preventing the widthwise movement of the printing sheet is provided the printing sheet meanders right and left, as a result of which stress is applied to the printing sheet 201, thus breaking the conveying perforations or creasing the printing sheet.
In order to overcome the above-described difficulty, means 209 for correcting the meandering movement of the printing sheet 201 (hereinafter referred to as "meandering movement correcting means 209") is provided. In this case, the meandering movement of the printing sheet 201 is detected by a sensor 211, and the contact pressure of the heating roll 207 and the pressure roll 208 is adjusted with respect to the axial direction of the heating roll 207. The meandering movement correcting means 209, as shown in FIG. 3, comprises: a left arm 301 and a right arm 302 supporting both ends of the pressure roll 208 which is abutted against the heating roll 207; and an electric motor 304 for driving the right and left arms through a wire 303. The right and left arms 302 and 301 are swung about their own fulcrums in the opposite directions by the electric motor 304 so that the contact pressure of the pressure roll 208 against the heating roll 207 is changed along the axial direction of the latter 207. That is, with the meandering movement correction means, the difference in peripheral speed between the ends of the heating roll 207 and the pressure roll 208 which is due to the deformation is utilized to correct the meandering movement of the printing sheet 201.
The correction of the meandering movement of the printing sheet 201 will be described with reference to FIG. 4 in more detail. When it is necessary to bend the direction of movement of the printing sheet 201 towards on side A in FIG. 4, the contact pressure of the heating roll 207 and the pressure roll 208 is made higher on the side A than on the other side B. In this case, in each of the rolls 207 and 208, the radius rA on the side A is smaller than that rB on the side B, and therefore the peripheral speed vrB on the side B is higher than the peripheral speed vrA on the side A (vrA &lt; vrB). Accordingly, the amount of movement of the printing sheet 201 is larger on the side B than on the side A; that is, the direction of movement of the printing sheet 201 is changed towards the side A. When, in contrast, it is required to bend the direction of movement of the printing sheet towards the side B, the contact pressure of the heating roll 207 and the pressure roll 208 is made lower on the side A than on the side B. In this case, the printing sheet 201 is shifted towards the side B.
The heating roll 207 is a both-ends-supported roll having a drive source, namely, an electric motor 101 on one end. Therefore, as shown in FIG. 1, for a very short period of time immediately after the printing sheet 201 is driven by the heating roll 207, because of the torsional twisting of the heating roll 207 the drive force is transmitted from the driving side A toward the non-driving side B in a twist mode. Accordingly, the angular speed of the heating roll 207 is higher on the driving side A than on the non-driving side B; that is, a condition wA &gt; wB is established. Accordingly, with the heating roll 207 uniform in diameter, the peripheral speed is higher on the side A than on the side B (vA &gt; vB). Under this condition, the amount of movement of the printing sheet 201 being conveyed is larger on the side A than on the side B, and therefore the direction of movement of the printing sheet 201 is bent towards the side B.
Thereafter, in a very short period of time, the drive force is transmitted uniformly in the heating roll 207, and the entire surface of the heating roll 207 becomes constant in peripheral speed. Under this condition, the amount of movement of the printing sheet 201 on the driving side A is equal to that of the printing sheet on the non-driving side B. Therefore, as shown in FIG. 5, a returning force which the puller 213 and the pressure roll 212 provide in response to the bending of the direction of movement of the printing sheet towards the non-driving side B and the rigidity of the printing sheet 201 in the widthwise direction shift the printing sheet 201 towards the driving side A.
On the other hand, the meandering movement correcting means 209 utilize the difference in peripheral speed between the right and left ends of the heating roll 207 and the pressure roll 208, to make the amount of movement of the printing sheet on the driving side equal to the amount of movement of the printing sheet on the non-driving side or vice versa. Therefore, in order to correct the meandering movement of the printing sheet, it is necessary to operate the meandering movement correcting means 209 for a period of time for which the printing sheet 201 is shifted to cover the difference in the amount of movement between the two sides A and B. Therefore, even if, when at the start of the conveyance of the printing sheet the sensor 211 detects the meandering movement of the printing sheet, the meandering movement correcting means is operated, it is impossible to completely eliminate the meandering movement of the printing sheet. In other words, immediately after when the conveyance of the printing sheet is started, the printing sheet is unstable being swung right and left.